Energy harvesting is the process by which ambient energy (energy derived from external sources) is captured and converted to electrical energy for powering devices. Remote sensor nodes must utilize energy harvesting of local, ambient energy sources for their power source in order to be untethered from the power grid or to avoid replacing batteries. Untethered sensor nodes have increased flexibility in their deployment. Local, ambient energy sources may include solar light, mechanical vibrations, mechanical strain, acoustic energy, temperature gradient, and electromagnetic energy at various frequencies including radio frequencies.
When solar light energy is not dependable (for example due to cloud cover) or is not available (for example due to embedded applications), the other energy sources must be considered for the energy harvesting applications. However, in many cases, ambient acoustic energy, RF energy, and thermal gradient-based energy are limited in magnitude and are often insufficient to power electronic circuits such as sensors, controllers, and transmitters. Although directed RF energy may possess sufficient energy to power such electronic circuits, the directed RF energy approach requires a focused RF source, not typically found in the local environment. Therefore, in certain situations, energy harvesting relies on mechanical vibration and strain.
Mechanical energy is typically harvested using a cantilever comprised of a proof mass (inertial mass) attached at an end of the cantilever. A cantilever-based vibration energy harvester relies on mechanical vibration at its base. This vibration causes a time varying acceleration of the base, which results in a mechanical strain in the cantilever beam due to the relative motion of the base with respect to the inertial proof mass. In a piezoelectric transduction-based cantilevered mechanical energy harvester, the mechanical strain in the cantilever is converted (harvested) into an electrical voltage by simultaneously straining a piezoelectric layer that is bonded to the cantilever structure. A piezoelectric transducer relies on mechanical strain where the strain generates a polarization charge density in the piezoelectric material and a resulting voltage across the electrodes spanning the piezoelectric layer. Such a cantilevered mechanical energy harvester that is excited by a time-varying base vibration is also termed a resonant energy harvester because the cantilever possesses a resonant frequency at which the cantilever tip deflection is maximum. One drawback of a cantilever-based vibration energy harvester is that the harvested energy decreases substantially if the vibration frequency does not closely match the resonant frequency of the cantilever.
Instead of a time-varying base mechanical vibration resulting in a time-varying strain in the cantilever and composite piezoelectric layer, a cantilever-based strain energy harvester may also convert a point load applied to its tip into a strain in a piezoelectric layer of the cantilever. However, cantilever-based strain energy harvesters have the disadvantage of a fixed base that impedes the application of the point load since the cantilever tip deflection stops when the tip touches the base. In addition, cantilever and other mechanical linkage methods of force transfer to the piezoelectric layer tend to be fragile and bulky due to the clearance necessary for the cantilever deflection or linkage displacement. Furthermore, the mechanical strain is proportional to the length of the cantilever, the magnitude of the proof mass, and the vibration amplitude. Therefore, miniaturization of the vibration energy harvester leads to a substantial decrease in the harvested energy. Hence, vibration-based energy harvesters have limitations in their applicability for small size applications.